Cusiana – A Giant in the Colombian Foothills

August 2013By Bernard C. Duval

American Association of Petroleum Geologists (AAPG)

This story, whose outcome was an important milestone for Total’s exploration at the time of discovery, can be seen as complementary to the Mahakam success story, described in the September 2011 Historical Highlights column.

The Mahakam story dealt with the rejuvenation of a mature hydrocarbon province, while this case study on Cusiana is about a frontier play.

Cusiana also is a fitting example of decision-making under high-risk conditions, miscellaneous difficulties and uncertainties inherent to frontier exploration.

Some of the messages derived from the case are still valid and applicable today.

♦ Message 1: Things do not always start in a grandiose way, and even “minor” actions can open the way to great opportunities.

Total had not been active in Colombia since the early 1970s. In 1986 an offer was made to acquire a local company’s minority interest in a joint venture operated by Exxon.

The main prospect was located in the Upper Magdalena Valley – a small, narrow but prolific basin developed between the Central and Eastern Andes in Colombia’s southern interior.

Drilling already had started when the deal was signed, and an oil discovery was made soon after at Yaguara (Los Mangos). This was not a huge field by international standards, but with some 60 million barrels was significant in the local context.

Although the contribution to the reserve base was modest, the success inspired optimism and impacted Total’s strategy in Colombia – and it triggered a wave of technical studies.

Business contacts soon materialized and a decisive farm-out opportunity arose.

♦ Message 2: Don’t hesitate to break away from an overwhelmingly discouraged common attitude. Just do your homework!

Triton, a small independent from Texas, had put together an offer to participate in a block named Santiago de la Atalayas, where they had recently shot a 2-D seismic program and defined a large anticline of significant closure (about 50 square kilometers) in the foothills of the Eastern Cordillera.

The farmout agreement would provide for the new partners to carry the cost of the first well in exchange for a stake in the block.

The play was not new – 17 wells had been drilled since the 1960s by different companies, some with big names, with no commercial success whatsoever. The lack of commercial success had caused frequent relinquishments and turnover of the different parties and operators.

Careful analysis of these wells showed that half of them had been abandoned for technical reasons linked to difficult drilling to depths close to 4,500 meters, and had not even reached the main target called the Mirador, a fluviatile sandstone of Eocene age.

Another observation was that all those wells had been drilled on poor seismic, almost unfit to generate any reliable mapping. In other words, the available data did not allow geologists and geophysicists to actually validate a prospect and test the play. However all the Mirador intersections had shown some sort of oil and gas shows.

Only one well flowed around 700 b/d (on the same structure as Cusiana!), obviously insufficient to warrant commerciality in Andean conditions.

The proposal was presented to the entire industry (more than 50 companies). However only two companies, BP and Total, were genuinely interested and convinced that the offer deserved serious in-depth consideration.

It is important to note that in case of discovery, Ecopetrol, Colombia’s national company, could exercise the right to take a 50 percent interest in the joint venture. Obviously, the petroleum community did not get excited, with good reason, about a risky and costly play seemingly tested on numerous locations.

Therefore it can be said at first glance that the glass could be half-empty (unsuccessful past programs) or half-full (abundant shows indicative of a working petroleum system?).

It is likely that many who looked at the opportunity had accepted the first scenario and did not go much deeper into the analysis.

The charge of the petroleum system was the first to be analyzed. It was clear from the subsurface and surface data that the Gacheta Formation of Middle Cretaceous age, more or less equivalent to the famous La Luna Formation of the Maracaibo Basin, the main source rock responsible for its giant accumulations, was an ideal anchor for a thermogenic petroleum system.

My modest contribution in the debate was that I had worked extensively in western Venezuela in the past and during that rich experience had spent time in the field, sampling the rocks of the Quebrada La Luna, the very site of definition.

Marching the rios is the only way to do field work in such jungle-covered terrain, because outcrops are only present along the banks. Spending days in the water, soaked with heavy rains and with all kinds of “nice” animals for company, is an experience you will never forget – not to mention discovering the greatest organic-rich source rock you might dream of as a petroleum geologist.

If such source was present at depth, with the right level of maturation, well … why not?

Geochemical modeling was conducted and showed that generation had probably started to take place in the area some three to four million years before the late Andean tectonics reactivated the existing Ceno-Mesozoic back-arc basin (six million years ago), creating the potential traps of the foothills. The models also reflected a possible continuation of the process during the inversion and until the present, an idea supported by the presence of light oil shows in some wells.

Note that the modeling techniques of the time did not allow geologists to evaluate the transformation ratio. Therefore, a serious doubt remained about the charge versus trapping chronology, not to count the inherent uncertainty of any modeling exercise.

To complete the charge evaluation we had to consider a major westward dipping fault striking north 30-40 degrees (like all structures of the foothills), as the most probable conduit (but also a potential leaking factor).

Different discussions with colleagues from different companies, years after the discovery, led to the thinking that this problematic question of a late trapping formation was a key factor in rejecting the proposal.

The reservoir was an element of concern too, for lack of significant production rates during past tests. The culprit could be depth and the associated diagenesis, which Total had encountered in other reservoirs in different areas of the world, with porosities even well above those measured on the Mirador (which were low, around 7-8 percent).

The trap itself was given a lot of attention. After reprocessing and time-to-depth conversion exercises, the geophysical team concluded that the structure had resisted well to all thinkable velocity assumptions. Therefore, the geophysical team was able to create a reliable map.

All in all, the accepted probability of success was 15 percent, with the critical points gravitating toward the timing and reservoir elements.

As for the volumetrics, the probabilistic distribution gave a mean value of 175 million bbls, and a maximum value of 350 million.

♦ Message 4: Having a wide-angle sweep vision of the geology – well beyond the local qualifiers of a prospect – may have a deciding impact on the evaluation. A willingness to take risk and an optimistic view of improving technical capabilities (e.g. drill better than was done in past operations) are key management drivers in frontier undertakings.

One could think at the time that getting a green light at the highest level of the company would be an uphill battle: The project had a low chance of success, drilling was difficult and costly, 17 wells had been non-commercial and there was a sensitive political situation with concerns about safety in the Cordillera (which limited access to the field).

Sharing the development with a partner was a prerequisite given the level of risk. BP was an obvious candidate given their interest in the project, and the two companies quickly agreed to join forces.

BP would be the operator while Total would contribute key geoscientists and engineers to the operating team, led by AAPG member Tony Hayward. Allowing the companies to communicate and share experience optimized both companies’ contributions.

It is time to mention that Cusiana, like all exploration ventures, was a collective undertaking. One member of the team in particular should be singled out and credited with the re-questioning of the “arrowed” thinking of the time, and advocating an optimistic concept: Jean Ferrat, a former IFP school student (same prom as your friendly scribe), later called ’L’Homme de Cusiana,” an inspired, gifted, intuitive geologist.

Jean was able to weigh on the positives and convince his colleagues and decision-makers that the half-full rather than half-empty glass should be given preference.

One of the arguments he made was linked to the existence of two oil fields, Bermejo and Orito, on the frontal part of the Eastern Cordillera. Although they were located far away from the proposed block (some 500 kilometers to the southwest, near the border with Ecuador), he insisted that at least somewhere in a similar fold-thrust-belt context the timing had been favorable, and that should help think of de-risking the related component.

To be honest, no uphill battle took place. When Jean and I met with Louis Deny, vice chairman of Total, who had a strong culture of the EP business, it was not difficult to convince him and get approval to go ahead.

♦ Message 5: The “maximum” volumetrics may even sometimes be exceeded by reality. The improbable can – and does – happen!

When the stage was set for the actual drilling operations, it was the start of a long story of difficulties of all kinds: stuck drill-bits, fishing, angle-drilling, questioning about the meaning of numerous shows encountered throughout the section, presence of a lot of gas (what to do with it?), doubts about VSP reflections and log interpretation (tight reservoirs or hydrocarbons?).

The Mirador sandstone finally was tested, yielding a disappointing flow of 1 000 b/d and six Mcf/d.

The teams did not get discouraged though, thinking that they could overcome likely damage caused by a long exposure of the reservoirs to the heavy mud. A second well, Cusiana 2A, located not far away from the first one, was needed to finally obtain a rate of 6,800 b/d and 19 Mcf/d, opening the way to commercial perspectives.

The actual reservoir quality was later attributed to the exceptional clean nature of a fine-grained sand, whose model could have been found on outcrops of the Forêt de Fontainebleau – 50 kilometers from Paris, a famous rock-climbing terrain!

It was astonishing to find oil and gas not only in the Mirador, but also down below in two sandy formations; the Barco and Guadalupe, of Paleocene and Upper Cretaceous age, respectively. All of the formations combined resulted in a 500-meter column of hydrocarbons and a potential areal extension and reserves much larger than expected.

In fact, the structure extended southward beyond the limits of the permit, to which acreage (Tauramena block) it was finally agreed to apply the same working interests as for Santiago de las Atalayas. The extension was then confirmed by a third well, Buenos-Aires-1.

In retrospect, one can think that charge was over-risked in a system with stacked generative layers, forebulge structures well developed at early stages, and possible, often underestimated, charge hysteresis.

♦ Message 6: Summarized in two words of wisdom and action – OPTIMISM and PERSEVERANCE! Which brings us to the conclusion – it’s THE HUMAN FACTOR, at all stages of the adventure, from concept to collective interplay before and during drilling, from the field to the different levels of decision-making.

A massive delineation program was then carried out, for which six heavy drilling rigs were soon mobilized (a rare effort at this stage of exploration), followed by development. As expected, Ecopetrol had exercised its back-in right once the field was formally declared commercial.

Another big (and happy) surprise was awaiting the joint venture. A second discovery was made seven kilometers north of Cusiana – a welcome sequence in a business that more often than once sets fire to great hopes.

The new field (Cupiagua) was found on a more complicated, intensely faulted structure, smaller in size and “boxy,” but containing a column of hydrocarbons three times larger than in Cusiana (1,500 meters).

The structure had been drilled in the past, with some light oil shows in the Mirador.

The Human Factor

All the phases of the deal and the operation itself bear witness of the importance of good geological thinking and technology: concept, processing, mapping, log interpretation, drilling, etc. My friend Jean played a key role in generating this adventure, but the accomplishment was not a cakewalk and would not have been possible without a symphony of skills and the strong valuable contribution from many colleagues and their collective interplay.

Also important was the responsibility assumed at different rank levels, as well as cooperation with the operator BP and partners, Ecopetrol and Triton.

I can only mention some of them on Total’s side, conscious of such an insufficient recognition: Jean Laherrère and members of his technical department, like Jean Francisque, who made the critical log analysis of the Mirador and recommended testing; Henri Orihuel, who helped interpret the VSP and conclude that 20ms of section remained to be drilled before reaching the frontal fault; Jean-François Mondy, geophysicist, whose structural interpretations were invaluable; Jean-Michel Fonck, whose team included Jean Ferrat; Jean-François Mugniot, who followed closely the appraisal and development operations; Michel Coudeyre, who was in charge of reserves evaluation within the BP team; and the engineers (Arnaud Peyredieu, Jean Ropers), who helped meet successfully the drilling challenges, advising on the use of oil mud to go through the thick shaly caprock.

At some time during the inauguration day in 1996, I was standing with Jean on a site from which we had a full view of the impressive facilities of Cusiana Field: a $6 billion investment to produce about 1,400 MMb, with a plateau close to 500,000 b/d (including Cupiagua), and I said to him:

“You see, Jean, all this huge machinery was born from an audacious concept and it was generated in your own mind. Warm congratulations, my friend!”

Many thanks to Jean Laherrère and Jean-François Mugniot, who have refreshed my memory and reviewed this article. Carson Caraway and Jean-Jacques Biteau have also kindly contributed to this review.

Bernard Duval, an AAPG Honorary member and Distinguished Achievement Award recipient, dedicated his life to exploration of petroleum systems. He is an Associate Professor at the IFP School (Institut Français du Pétrole), Rueil-Malmaison, France.

Historical Highlights

Hans Krause is an AAPG Honorary Member, Distinguished Service Award winner and former chair of the AAPG History of Petroleum Geology Committee.

Historical Highlights

A history-based series, Historical Highlights is an ongoing EXPLORER series that celebrates the "eureka" moments of petroleum geology, the rise of key concepts, the discoveries that made a difference, the perseverance and ingenuity of our colleagues – and/or their luck! – through stories that emphasize the anecdotes, the good yarns and the human interest side of our E&P profession. If you have such a story – and who doesn't? – and you'd like to share it with your fellow AAPG Members, contact the editor.

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